On the '05 the temp gauge on the dash is run by the computer.
The computer uses an ECT(engine coolant temp) sensor to get the temp.
ECT sensors are not expensive so I would just replace it, 3rd party are under $15 usually
Should be located right next to thermostat housing, it is a 2 wire sensor
Yes, if ECT sensor is reporting cold engine the computer needs to run the engine Rich, Choke mode, so MPG would be down.
But computer should also give you a code that tells you that engine is not going into Closed Loop(warming up) after about 5 minute of running.
If you have codes you should post them.
Many people do not understand what codes mean, even some mechanics, they are not to be taken literally.
i.e. Lean code does NOT mean engine is running lean, and Rich code does NOT mean engine is running Rich.
Both mean the computers base calculation for the 14:1 air:fuel ratio(MAF sensor data) are not being confirmed by the upstream O2 sensors feedback(data)
Codes do help point to problems but they should be looked at as a whole, all the codes.
Often clearing the codes and then driving for 2 or 3 days and then re-reading them will further help to get to the heart of the problem.
Stock Thermostat should be 190-195degF, parts guys often give you a 180degF, cooler is not better.
Also run 50/50 coolant/water mix, all coolant will run way too cold
To test thermostat start cold engine, feel the upper rad hose, it will be cold
let engine idle for 2 or 3 minutes, feel upper hose again, it should still be cold, if not then thermostat is open and it shouldn't be
After 5 to 8 minutes thermostat should start to open and upper hose will start to warm up.
The Thermostat is the only minimum temp control on the engine, so engine actually running cold has to be a thermostat issue, engine should run with coolant at 200degF so quite warm to the touch on upper rad hose after full warm up.
Not sure what you mean in this: "when I turn it on, it draws down the engine to the point that I have to turn it off immediately"
You mean the engine starts to stall or the temp gauge goes down more?
Last edited by RonD; 10-19-2015 at 11:11 AM.